Wood-resin composite products, such as plywood, laminated veneer lumber (LVL), oriented strand lumber (OSL), oriented strand board (OSB), waferboard, particleboard, medium density fiber board, hardboard and the like, traditionally have been made by combining a binder resin with wood components to form a stack or mat which is then consolidated in a hot platen press to cure the binder resin.
In many cases, the binder resin is a phenol-formaldehyde resin and is supplied in the form of a powder. Manufacturing OSB and waferboard are applications where phenol-formaldehyde resin powders, in particular, find a significant level of use. The powdered resins have a number of advantages over liquid resins in such applications, such as: (a) simple and less expensive equipment often can be used for handling and blending the resin with the wood furnish; (b) powdered resins are easy to blend onto wood wafers and strands, usually achieving a more uniform resin distribution; and (c) powdered resins generally have longer storage lives, so inventory control is easier to manage.
Historically, such phenol-formaldehyde resin powders are prepared by spray-drying a liquid phenol-formaldehyde precursor resin made under alkaline conditions (i.e., a resole resin) in the presence of a sodium hydroxide catalyst. Such resins are fast curing and provide wood composites of high strength.
The present invention represents a departure from the conventional approach of using sodium hydroxide as the sole source of alkalinity when synthesizing such powdered resins. In particular, applicants have discovered that a phenol-formaldehyde resin catalyzed solely using sodium hydroxide is not the best choice under all wood composite manufacturing conditions. Depending on manufacturing conditions, wood composites of improved properties can be prepared using a powdered phenol-formaldehyde resole resin that has been prepared in the presence of a source of calcium ions.